(a) Field of the invention:
The present invention relates to a connecting optical system to be used for transmitting illumination light through endoscopes (especially non-flexible type) and, more specifically to a connecting optical system to be used especially when two light guides are used in mutually connected condition.
(b) Description of the prior art:
Light guides composed of optical fiber bundles or the similar material are used, for example, as illumination systems for endoscopes. In such an illumination system, a light guide 2 arranged on the side of a light source 1 is sometimes connected, at a point indicated by the reference numeral 5, to another light guide arranged on the object side in an endoscope as shown in FIG. 1. The reason to connect two light guides as shown in FIG. 1 is to eliminate operating inconvenience, i.e., disconnecting a long light guide from the light source and replacing it with another, for example, to replace several types of scopes during medical operation which may be required in case of an integral type of light guide consisting of a continuous section corresponding to both the light guide 2 arranged on the side of the light source and the light guide 4 arranged on the object side. In case of a dual-section type of light guide which consists of two light guides which are connected to each other, however, loss of light in the connecting system poses a problem.
When light is transmitted by using a light guide, loss of luminous intensity generally results from the fact that all the light falling on the end surface of incidence of the light guide is not transmitted since the core portion that allows the light to pass has a sectional area (effective sectional area) of 50 to 70% of the sectional area of the light guide. In case of the dual-section type of light guide mentioned above, loss of luminous intensity is more remarkable than that in the integral type of light guide and light is attenuated to 25% to 50% due to loss of luminous intensity in the connecting optical system. In an endoscope either of the integral or dual-section type, the section corresponding to the light guide 2 arranged on the side of the light source shown in FIG. 1 is prolonged for assuring convenient manipulation by the person who is being engaged in medical operation. When the light guide is long as described above, the angular distribution characteristic (ratio of luminous intensity of a ray incident at a given angle relative to that of a ray incident in parallel with the optical axis) of the light emerging from the light guide is degraded while it was passing through the light guide even if the characteristic is favorable before the light is incident onto the light guide from the light source. In case of the example illustrated in FIG. 1, the angular distribution characteristic of the light is degraded chiefly by the light guide 2 arranged on the side of the light source. As the conventional connecting optical system for light guides of endoscopes, there are known the direct connection type shown in FIG. 2 and another type shown in FIG. 3 in which the emerging end surface 2a of the light guide 2 arranged on the side of the light source is used as a secondary light source whose image is formed by a lens system 6 on the incident end surface 4a of the light guide 4 arranged on the object side.
Before discussing the angular distribution characteristics of these conventional types of connecting optical systems, angular distribution characteristic at the emerging end surface of the light guide 2 arranged on the side of the light source will be described. The angular distribution characteristic experimentally determined at the emerging end surface of the light guide 2 is represented by curve a shown in FIG. 5. This curve is similar to curve b also shown in FIG. 5 which is a quadratic curve of I (sin .theta.)=-a sin.sup.2 .theta.+b wherein sin .theta. is taken as a variable. When the emerging end surface of the light guide arranged on the side of the light source is used as the secondary light source, it is therefore possible to represent the angular distribution characteristic on said end surface in the form of a quadratic curve. Further, plane luminous intensity distribution in the radial direction (2r.sub.1) of the light guide is uniform due to the characteristic of the light guide. Therefore, plane luminous intensity distribution and angular distribution characteristic in the radial direction of the secondary light source are as illustrated in FIG. 6A and FIG. 6B respectively.
In case of the dual-section type of light guide consisting of two light guides which are connected directly as shown in FIG. 2, the plane luminous intensity distribution and angular distribution characteristic on the light guide 4 arranged on the object side are the same as those of the secondary light source, and therefore as shown in FIG. 7A and FIG. 7B respectively.
Furthermore, in case of the light guide in which image of the secondary light source on the end surface of the light guide arranged on the side of the light source is formed on the incident end surface of the light guide arranged on the object side by using such a lens system as shown in FIG. 3, actually measured values of the light distribution characteristic are as represented by curve b illustrated in FIG. 10.
As is clear from these data, in the directly connected type light guide among the conventional types of light guides, the illumination light is incident onto the light guide arranged on the object side while keeping the plane luminous intensity distribution and angular distribution characteristic on the secondary light source. Therefore, the illumination light having angular distribution characteristic degraded by the long light guide is transmitted as it is, making it impossible to illuminate with favorable angular distribution characteristic. In case of the light guide in which an image of the secondary light source is projected by using the lens system shown in FIG. 3, the angular distribution characteristic is somewhat improved but is not sufficiently favorable as is judged from the curve b shown in FIG. 10.